Common C Errors

Common Compiler Errors

Know the exact messages and you'll fix bugs much faster.

Missing semicolon

expected ';' ...

Forgot at the end of a statement.

Implicit declaration

warning: implicit declaration of function 'print'

Called a function without a prototype/definition. Usually a missing header.

Wrong specifier

printf("%s", 42);

Passing an int where %s expects a string - garbage or crash.

Buffer overflow

char name[5];
strcpy(name, "This is much too long!");

Writing past the array bounds is undefined behavior.

Undefined behavior

• Reading uninitialized variables.
• Using freed pointers.
• Signed integer overflow.

May seem fine, crash later, or behave randomly.

TL;DR

• Read the compiler's message; start at the first error.
• Missing semicolon: add one.
• Wrong header: use the correct one / prototype.
• Watch array sizes to avoid overflow.
• Undefined behavior is a bug even if it works.